A standard hand-operated device for transferring a film from a carrier tape to a substrate has a housing made of two parts that are pivoted together. The housing has two rotatable spindles coupled to each other by a slip-permitting transmission. A cartridge is held in this housing and has fitting on the pivot pins a supply spool and a takeup spool for the tape and an applicator element at one end. The tape passes from the supply spool over the applicator element which is used to press the tape against the substrate for transfer of the film from the tape to the substrate. After the film is stripped from the tape, this tape is wound up on the takeup spool which itself is provided with a one-way brake allowing it to rotate only in one direction. Such an arrangement is described in commonly owned U.S. Pat. Nos. 4,849,064 and 4,853,074.
The film is often an adhesive layer acting in effect like a double-faced tape. After the adhesive film is stripped from the carrier tape, same passes back over the supply of the tape on the spool, inherently sticking somewhat to same although the tape itself has such a smooth surface that adherence is weak, so that the tape will remain fairly snug along its path over the applicator element. When, however, the film is an opaque cover-up having a face covered with contact adhesive, the outer surface of the supply spool is constituted by the smooth nonsticky face of this cover-up film so that until the cartridge is mounted in the holder, it is possible for the tape to loosen and form a slack loop because the only thing preventing reverse rotation of the supply spool is the one-way brake of the takeup spool which is effective through gearing and a slip clutch on the supply spool. Unless the user meticulously tightens the tape before loading the cartridge in the holder, this loose loop will make subsequent application of the film to a substrate difficult or impossible.
Some means is normally provided to keep some tension in the tape so that it can be applied accurately, and also to create some tension in the tape when same has loosened as described above. This is most easily done by setting the transmission ratio of the gearing driving the takeup spool from the supply spool such that the takeup spool always rotates at least a little bit faster than the supply spool, and by providing a slip clutch in this transmission that itself creates the desired tension. The diameter of the supply spool decreases and that of the takeup spool increases proportionately as the supply is used up, so that the slip must be at a minimal level at the very end of the life of the cartridge, when the supply is exhausted. If the tension is too great at the end, the tape will snap. On the other hand the varying lever arms created by the changing diameters of the supply and takeup spools means that the tension created by the slip clutch invariably provided between the large-diameter supply spool and its coaxial drive gear will inherently increase as the supply is used up. Thus to start with the tension will be very low so that a loose loop in the tape will not be eliminated or one can even be created. Of course once the tape is used up the increasing tension will normally rectify the problem, even if, for instance, a cartridge is taken off the holder and then put back on with its tape somewhat loose.
In order to prevent the tape in a cartridge for a typewriter or printer from loosening prior to installation it is known to fit the legs of a disposable U-shaped piece of cardboard or the like into the spools, so as to prevent same from rotating prior to installation. If the cartridge is not installed in the holder immediately after this element is removed, however, the tape can loosen, or if one cartridge is swapped out for another to change film colors or the like, loosening can occur, as the retaining strip is normally discarded.
Another known system has a brake that acts continuously on one or both of the spools. Such a brake is typically formed as an integral elastically deformable tab that projects from the cartridge and bears on the spool. The friction between this tab and the spool therefore brakes the spool and prevents it from rotating freely. Unfortunately with such a system the braking force normally increases as the tab wears and the surface area with which it bears on the spool increases. Such increased braking force can result in breakage of the tape toward the end of the life of the cartridge. In addition while this increasing braking action is not significant in a motor-driven system, it is noticeable in a hand-held dispenser and makes such a dispenser harder to use, as sometimes it takes considerable force to apply a film with it and other times it takes relatively little force.